1. Field of the Invention
The invention relates to advanced adsorbents useful in pressure swing adsorption processes. More particularly, it relates to the recovery of lithium used for the production of the lithium exchanged forms of such advanced adsorbents.
2. Description of the Prior Art
In chemical processing, refinery, metal production and other industrial applications, purified gas streams are employed for a variety of processing purposes. For example, high purity oxygen is used in chemical processing, steel mills, paper mills, and in lead and gas production operations. Oxygen and nitrogen are produced from air, typically by cryogenic distillation. While such cryogenic processing can be very efficient, particularly when conducted in large size plants, it nevertheless requires complex and costly equipment.
Pressure swing adsorption (PSA) processes have also been used to separate and purify gases, as for the production of oxygen generally in relatively smaller-sized operations where the use of cryogenic air separation may not be economically feasible. Many commonly available adsorbents, particularly the class of materials known as molecular sieves, selectively adsorb nitrogen more strongly then oxygen, and this preferential adsorption is the basis of a variety of PSA processes that have been developed for the separation of air to produce oxygen and nitrogen product gas.
The Chao U.S. Pat. No. 4,859,217, discloses the lithium cation forms of zeolite X, particularly the forms in which the framework Si/Al molar ratio is from about 2.0 to about 3.0, preferably from 2.0 to 2.5, wherein at least about 88%, preferably at least 90%, and more preferably at least 95%, of the AlO.sub.2.sup.- tetrahedral units are associated with lithium cations. In the Chao et al. patent, U.S. Pat. No. 5,174,979, lithium/alkaline earth metal zeolites of the X and A type are disclosed, with lithium to earth metal ratios of 95:5 to 50:50, and 10:90 to 70:30, respectively.
Such mixed cation materials are also well suited to PSA-air separation operations and have high thermal stability characteristics.
Li+ zeolites are commonly prepared from the corresponding Na+ zeolites by ion exchange. A concentrated aqueous solution of Li+Cl- is passed through a column containing the Na+ zeolite. The Na+ ions are displaced by the Li+ ions to produce the desired Li+ zeolite. Since zeolites generally have a greater affinity for the Na+ ion than for the Li+ ion, a considerable quantity of strong Li+Cl- solution is required, and the spent liquor from such ion-exchange operations contains a high concentration of both Na+ and Li+ ions. The contained lithium is too valuable simply to waste, and a process of evaporative concentration and fractional crystallization can be employed to remove by precipitation most of the NaCl, leaving a concentrated Li+Cl- solution that can be reused in the ion-exchange process. Thus, the bulk of the lithium is retained in the ion-exchange system.
After conversion, the lithium-exchanged adsorbent must be washed and dried prior to use in PSA operations. The spent wash water contains Li+ ions, which are lost in the discharged waste water. In addition, other Li+ is lost in off-specification exchanged zeolite. This Li+ can be released into solution by displacement with Na+. Still more Li+ is lost in the Li+-rich solution that adheres to the precipitated NaCl crystals. This latter lithium could be recovered in solution by washing the crystals in soft water.
The concentrated Li+ solutions resulting from desired ion-exchange operations for the production of lithium exchanged zeolites can be economically treated for the recovery of lithium in said recrystallization plant. However, dilute Li+-containing solutions would require too much energy consumption in order to be treated in this manner. As a result, such dilute Li+-containing solutions are often discharged as waste. Unlike NaCl, which is abundant and cheap, LiCl is a rare and costly commodity. Thus, the Li+ lost to waste adds appreciably to the overall cost of producing lithium exchanged zeolite. There is a strong desire and economic need in the art to recover such Li otherwise lost to waste so as to reduce the overall processing cost of producing lithium exchanged zeolite.
It is an object of the invention to provide a process for the recovery of lithium from the dilute Li+-containing solutions obtained in the production of lithium exchanged adsorbents.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.